Changing the hue of texture image
You can definitely do a lot of editing in the compositor and then save the results, and then reload in as a new texture. Here in this simple example I am using the Compositor window and bringing my painting in that is show in the lower right hand uv image editor in it's first state in monotone value. I use an Image Node to bring it into the compositor, and then direct it to a Viewer node for output, using the Backdrop to see it there. In the lower left uv image editor, you can see I have chosen 'Viewer' output to see the same as the backdrop, and that is where the result can be saved by choosing 'Save Image As' from the Image options in the header. I have used a simple Color Mix node set to 'Color' and chosen a light blue and not so full strength. You can use all kinds of filters and correction nodes to edit your images there. Just don't confuse the Viewer node with the Composite output node - the viewer is what you need to send the output to the image editor.